The present invention relates to a method for providing an optical signal appearing at the rear of a vehicle to warn the driver of a following vehicle.
Such an optical signal is, for instance, a brake light indicating a brake pedal actuation. Brake lights are prescribed for all automotive vehicles. In some countries, for four-wheeled vehicles even a third, central, brake light is required. The purpose is to provide the following traffic with the information of an imminent or ongoing deceleration.
Nowadays, a majority of vehicles is equipped with an electronic control system comprising at least one of the following functions: antilock brake system (ABS), traction control system (TCS), automatic yaw control (AYC). All these functions influence the behavior of the vehicle without a respective driver's action. While ABS reduces the brake force of individual wheels during a pedal actuated braking operation when the applied brake force threatens to lock a wheel, the other electronic control systems are capable of applying a brake force to selected or all wheels under certain conditions according to preestablished control algorithms. Thus, an deceleration of the vehicle can occur, either due to an improved brake efficiency during a pedal-actuated brake operation supported by ABS or due to a so-called "active" brake operation through TCS or AYC operation provoked by excessive traction slip or unstable vehicle behavior during cornering. In each case, such vehicle deceleration may not be expected by the driver of a vehicle behind the decelerating vehicle.
To provide a warning for the driver of a following vehicle, German patent No. 43 05 186 proposes an additional warning light at the rear of a vehicle to be switched on dependent on a hazard value. The hazard value is derived by time integration of the current vehicle deceleration over a certain time period. If the hazard value exceeds a certain threshold, the warning light is activated and remains shining until the hazard value has decreased again. During ABS control, the deceleration is multiplied by an amplification factor so that the threshold is reached earlier than without ABS intervention.
According to German patent No. 43 05 186, for deriving the hazard value, the vehicle is equipped with an additional accelerometer for measuring the vehicle deceleration, which causes additional costs.
While a deceleration-dependent warning is an asset compared to only pedal-actuated brake lights, it is disadvantageous that, on the one hand, the warning light will always shine up even when a deceleration occurs entirely due to driving uphill or to braking forces that are normal for the circumstances and can be easily foreseen by following drivers. In such cases, no additional warning is needed. On the other hand, no warning will be given by known systems in situations that require an intervention of an AYC or TCS. For instance, if a vehicle is equipped with an AYC, an inadequate cornering maneuver can be compensated by selective brake application of the inner rear wheel in case of understeering and of the outer front wheel in case of oversteering. By the driver of a following vehicle driving in the same manner, but without such an automatic control function, the cornering maneuver is approached without advance notice of a dangerous condition, and the driver has no reason to reduce speed since the vehicle ahead manages the situation without a sign of instability. For the following vehicle, however, the same behavior can be fatal. Similar considerations apply for low-friction road surfaces when only an intervention of TCS can maintain a sufficient traction.